1. Technical Field
This invention relates to the art of making catalysts for treating automotive emissions, and more particularly to three-way catalysts that have high conversion efficiency for hydrocarbons, carbon monoxide, and nitrogen oxides
2. Discussion of the Prior Art
Precious metals, including palladium, have been used as active catalyst materials in automotive emission devices. However, palladium, by itself, as the dominant or sole catalyst material, is limited in its effectiveness to promote the reduction of nitrogen oxide compounds at high temperatures and is not optimum in promoting oxidation of hydrocarbons at such temperatures. An example of palladium usage is disclosed in U.S. Pat. No. 4,123,391. A variety of cost-increasing alternatives have been used to augment the capabilities of palladium, primarily by the use of other precious metals.
This invention has discovered that small amounts of titania (a nonprecious metal) uniquely applied will synergistically cooperate with palladium to enhance the total catalytic effect for oxidation and reduction, particularly at high temperatures. Titania has not been used heretofore as a catalyst enhancing ingredient. Titania has been incorporated in catalyst support materials for purposes of stabilizing such support materials, but prevented from entering into the catalytic process by coverings of other catalytically active ingredients over the titania (see U.S. Pat. Nos. 4,504,598 and 4,123,391). In some cases, titania has been layered on the support in large amounts prior to being covered with the catalytically active material (see U.S. Pat. No. 4,350,613).
Straight titania has been slurried onto prevalent precious metal materials (including palladium) in a thin uncovered continuous film (of about 0.003-0.01 inches thick) to make an oxidation catalyst more lead tolerant (see U.S. Pat. No. 4,650,782). Such catalyst construction was not intended for enhancement of reduction processes, did not function as a reducing catalyst, and, because of its continuity, found no special catalytic synergism between titania and palladium.